Space vehicles customarily employ heat pipes for achieving heat transfer from temperature-sensitive components such as electronic circuits. Heat pipes are passive devices with no moving parts and are therefore very reliable. However, because the wick in a heat pipe must be continuous and extend over its entire length between the evaporator and condenser, fluid pressure drops become excessive with power levels beyond a few kilowatts. For larger loads, mechanically pumped two-phase thermal control systems (TCS) are ideal and are considered for installations which are manned. This is due to the fact that the systems use mechanical components such as pumps and valves, which consume vehicle electric power and require maintenance and redundancy to achieve long orbital life.
In the case of space platforms, power requirements (5 to 15 kw) are too large to be handled by conventional heat pipes and too small to require a mechanically pumped two-phase loop. The prior art includes a device known as a capillary pumped loop (CPL) which has been recently developed for NASA by the OAO Corporation of Maryland. Such a device could satisfy the needs of moderate power space platforms. The CPL utilizes a very fine wick structure to provide a high pumping potential. Because the wick is only located in the evaporator, the rest of the loop can be made using simple (unwicked) tubing so that the system pressure drop will be small. An electrically heated accumulator is needed to allow loop operation at the desired temperature. Thus, the CPL offers a reliable moderate power capability thermal control system that is passive, i.e. having no moving parts, and operates with the same vibration-free characteristic as conventional heat pipes.